This invention relates to bearings, and, more particularly, to bearings used at elevated temperatures and in adverse environments.
In a gas turbine (also sometimes termed a "jet") engine, air is drawn into the front of the engine and compressed by a shaft-mounted compressor. The compressed air is mixed with fuel, and the mixture is burned in a combustor. The resulting hot exhaust gases are passed through a turbine that is mounted on the same shaft as the compressor, and then flow out of the rear of the engine. The rotation of the turbine causes the main shaft to turn, powering the compressor.
The main shaft upon which the compressor and turbine are mounted is supported on several sets of bearings. Although they are not exposed to the extremely high temperatures experienced by the turbine blades and vanes, these bearings are subjected to intermediate service temperatures of up to about 350.degree. F. in current engines.
The contact surfaces of the bearings must have high hardness, wear resistance, strength, and fracture toughness, at the service temperature of the bearings. The bearings are currently made of specialty tool steels such an M5O steel, which meet the indicated requirements at temperatures of up to about 600.degree. F. At higher temperatures, however, the bearings suffer reductions in these properties as well as increased susceptibility to environmental damage such as oxidation and corrosion. It is also difficult to maintain tolerances at the higher temperatures, due to thermal expansion mismatch between the bearing material and the shaft material.
There is an economic incentive to develop gas turbine engines that operate at ever-higher temperatures, because the fuel efficiency of the engine increases with increasing combustion gas temperatures. As the operating temperature of the engine is increased due to innovations in the design and materials used in the turbines and other hot-section components, it is desirable to operate the main shaft and its bearings at ever-higher temperatures, to reduce the need for excessive cooling of components within the engine and to permit innovations in shaft and bearing design.
There is therefore a continuing need for improved bearing materials for use at elevated temperatures in jet engines, as well as other applications. The present invention fulfills this need.